The present invention relates to the field of micromachined devices and, more particularly, to dimensional control of structures within micromachined devices.
"Micromachined device," as the term is used herein, refers to a device containing a three-dimensional structure manufactured using the same photolithographic techniques and batch processing as for integrated circuits. Micromachined structures are frequently used as sensors or actuators and, in some existing applications, are used to detect and measure acceleration with piezoresistors or capacitors. In the latter case, a differential capacitor is typically used.
A differential-capacitor based acceleration sensor includes three primary micromachined elements; a central member, capacitor plates and support springs. The member, supported by springs, is positioned midway between two plates so that one capacitor is formed by a first plate and the member and a second (and equal) capacitor is formed by a second plate and the member. To maximize the sensor capacitance, the member may contain numerous fingers that are interleaved between fingers from the two plates. Various shapes and arrangements of the capacitor plates may be used. Examples of such devices are provided in commonly-owned U.S. Pat. No. 5,345,824 and U.S. patent application Ser. No. 08/347,795 now U.S. Pat. No. 5,565,625, both of which are hereby expressly incorporated by reference in their entirety for all purposes.
The sensitivity of a micromachined sensor is determined by a variety of factors, including sensor capacitance, spring constant ("k"), mass of certain elements (e.g., central member), polysilicon thickness, parasitic capacitance and variations in crystal orientation in polysilicon. Among these, spring constant and sensor capacitance are the dominant factors. The sensitivity of a micromachined sensor may vary significantly due to manufacturing variations which alter dimensions of micromachined structures within the sensor. Accordingly, some post-manufacturing calibration is typically required. Effective calibration requires an accurate determination of sensitivity, which is largely determined by accurate estimations of spring constant and sensor capacitance values.
Although the springs and capacitors of a micromachined sensor are manufactured using the same processes, variations in these processes do not necessarily affect the different structures disposed within these components the same way (i.e., each structure may vary independently in size). Such disparate dimensional variation between physically and/or functionally different structures makes it difficult to accurately estimate component values and, thereby, predict device performance.
Accordingly, a new micromachined device is required that facilitates correlation of dimensional variation between micromachined structures.